Many water plants which are used to produce drinking water utilize membrane filters (e.g. micro-filtration, ultra-filtration, and nano-filtration). Typical ultra-filtration membranes have an effective pore size of less than 0.1 micron which means that not only do they completely remove most bacteriological pathogens, but they also have the capability to filter out most viruses. The California Department of Health and other state and federal agencies are considering means by which they can offer virus removal credit to water plants which adopt ultra-filtration technology. If adopted, water plants will be able to save very significant amounts of money by avoiding the costs associated with chemical disinfection of drinking water.
A serious disadvantage associated with the use of filtration membranes is the periodic failure or rupture of a membrane. Typically there are thousands of relatively small diameter elongated membrane fibers contained in a parallel arrangement in a single cartridge, with numerous cartridges (e.g. 1000 or more in racks of 20 to 50) being used simultaneously in a single water plant. Presently, the operator of the plant periodically (e.g. every four hours or so) takes each rack off-line and passes air through one end while the other end is submerged in water. Below a certain air pressure, air should not be able to pass through a membrane, unless the membrane has been punctured or has ruptured. If the membrane has ruptured or been punctured, a stream of bubbles will be detected from that membrane at the submerged end. The failed membrane can then be identified and mechanically plugged, after which the cartridge can be put back into service. The main problem with this approach is that it allows for a four to six hour period during which undesirable material could pass through the cartridge and enter into the effluent water.
Although manufacturers have provided turbidimeters and particle counters for on-line monitoring of water quality, those instruments generally lack sensitivity to sub-micron particles. Furthermore, a full scale water plant would require several hundred membrane cartridges, and the instrumentation costs for monitoring the effluent of each membrane would be prohibitive.
There has not heretofore been provided a system for accurately and efficiently monitoring the effluent of multiple membranes used for ultra-filtration of water.